1. Field of the Invention
The present invention relates to a spool valve, and more particularly, to a spool valve structure which can compensate flow force applied to the spool in a sleeve by changing the shape of a spool rod.
2. Description of the Background Art
A spool valve indicates a valve which opens and closes a channel by a spool which moves toward a longitudinal direction of the spool rod in a sleeve.
FIG. 1 is a cross-sectional view showing a structure of a general spool valve in accordance with the conventional art.
As shown, the conventional spool valve comprises a supply channel 11(11a, 11b); a sleeve 10 having a return channel 12; and a spool 20 having a plurality of spool lands 21 connected to one another as a unit by a spool rod 22. The spool 20 reciprocates in the sleeve 10 toward the longitudinal direction of the spool rod, thereby opening and closing the channels 11 and 12.
In the sleeve 10, one or two control channel 13(13a, 13b) is formed, which is called as a three-way control valve or a four-way control valve according to the number of entire channels.
Herein, a flow of fluid which passes the spool valve will be explained.
The fluid supplied through the supply channel 11 passes a metering orifice formed by a narrow space between an end portion of the supply channel 11 and the spool land 21, thereby forming a fast flow. The fast flow is reflected by the spool rod 22 and then flows out through the control channel 13 and or the return channel 12.
A magnitude of the flow force applied to the spool 20 by the fluid is equal to that of a force by a pressure applied to a wall of the spool land 21, and a direction of the flow force is opposite to that which the spool moves, that is, a direction which the metering orifice is closed.
The flow force will be expressed as follows by forces applied to each part A, B, C, D, and E of the spool valve shown in FIG. 1Fflow force=−FA+FC−FD+FE
When the flow force becomes large, a force required to move the spool of the valve also becomes great, so that a size of the entire valve or a capacity of a driving motor has to be increased.
Accordingly, many methods for reducing or perishing the flow force of a steady state have been proposed. However, only some parts of the methods were successful and the other parts were impractical.
In the American patent of a number U.S. Pat. No. 4,220,178, a method for changing a shape of the spool rod (a shank of the spool) of a hydraulic control valve is proposed to change an exchange of a movement amount between the spool and a high pressure fluid so that many kinds of dynamic forces applied to the spool be balanced one another.
The shank is formed to have an angle smaller than an inflow angle in order to reduce the flow force which can be generated at the spool.
However, said method accompanies one disadvantage that a maximum diameter of the spool land is increased when compared with a structure of the conventional spool and has a problem that a shape of an exit ramp is not easily and precisely fabricated.
Also, in the American patent of a number U.S. Pat. No. 5,944,042, a method for compensating the flow force by using a re-circulation land is proposed.
A hydrostatic bearing is formed in the sleeve and a groove is formed in the spool land to compensate the flow force by a recirculation generated by the structure.
However, said method requires a structure change of the sleeve itself, so that a fabricating cost is excessively increased and a precise fabrication is impossible.
Also, in the American patent of a number U.S. Pat. No. 3,123,335, a method for compensating the flow force by forming a notch at the spool land is proposed. However, said method requires a structure change of the sleeve itself, thereby excessively increasing a fabricating cost and not being able to fabricate accurately.